1. Field of the Invention
The present invention relates to a motor driving apparatus that converts an alternating-current power supplied from the three-phase alternating current input into a direct-current power and outputs the same to the direct-current link, and thereafter further converts the same into an alternating-current power for driving the motor and supplies the same to the motor. In particular, the present invention relates to a motor driving apparatus including a life determining unit for determining the life of a direct-current capacitor provided in the direct-current link.
2. Description of the Related Art
In a motor driving apparatus for driving a motor in machine tools, industrial machines, forging machines, injection molding machines, or various types of robots, an alternating-current power inputted from the alternating-current power source is first converted into a direct-current power by means of a converter, and then the resulting direct-current power is converted into an alternating-current power by means of an inverter, and the resulting alternating-current power is used as a driving power of the motor.
FIG. 4 is a diagram depicting a configuration of a general motor driving apparatus. The motor driving apparatus 100 includes a converter 101 converting an alternating-current power from a commercial three-phase alternating-current power source (hereinafter simply “alternating-current power source”) 103 into a direct-current power, and an inverter 102 converting a direct-current power outputted from the converter 101 into an alternating-current power of a desired frequency to be supplied as a driving power of the motor 104 or converting an alternating-current power regenerated by the motor 104 into a direct-current power, and controls the speed, torque of the motor 104 connected to the alternating-current side of the inverter 102, or the position of the rotor. The converter 101 is connected to the inverter 102 via a direct-current link (DC link). A direct-current capacitor (DC link capacitor) 105 is provided in the direct-current link, for smoothing the direct-current output of the converter 101.
Generally speaking, when there are a plurality of driving shafts (feed shaft and main shaft), a plurality of motors are provided, too, for driving each driving shaft. The inverters are provided in the same number as the number of motors and are connected in parallel, for driving and controlling the motors by supplying a driving power to motors individually, which are provided for the plurality of driving shafts, respectively. Direct-current capacitors are provided at the direct-current input side of inverters, respectively. On the contrary, a single converter is often provided for a plurality of inverters, for reducing the cost and the occupied space of the motor driving apparatus. Note that for simplifying the drawing, FIG. 4 depicts a single motor 104, and therefore the number of inverters 102 is one.
The direct-current capacitor provided in the direct-current link between the converter and the inverter is generally known as a component having a limited life whose electrostatic capacity (hereinafter simply “capacity”) decreases by repetition of charge and discharge. When the capacity of the direct-current capacitor decreases, the ripple current running through the direct-current link increases, to increase the fluctuation in direct-current voltage increases, which is problematic. The direct-current capacitor determined to have ended its life needs to be exchanged. In view of the above, it is important to determine whether the life of a direct-current capacitor is ended or not by accurately grasping the capacity of the direct-current capacitor.
For example, as described in Japanese Unexamined Patent Publication No. 2000-152643, an instrument is known that determines a life of a direct-current capacitor using a time integration value of a charge current of the direct-current capacitor at the initial charge and the voltage value of the direct-current capacitor.
FIG. 5 depicts an overview explaining the invention described in Japanese Unexamined Patent Publication No. 2000-152643. In this explanation, an example in which a plurality of motors are provided to drive n driving shafts (n is a natural number equal to or greater than 2) is taken. N inverters are provided to supply a driving power to n motors, and n direct-current capacitors are respectively provided for n inverters. By representing each set made up of a motor, an inverter connected to this motor and a direct-current capacitor connected to this inverter as instruments 120-1, 120-2, . . . , 120-n, n instruments 120-1, 120-2, . . . , 120-n are connected in parallel as depicted in FIG. 5. Note that so as to simplify the drawing, FIG. 5 does not depict motors and inverters for the instruments 120-1, 120-2, . . . , 120-n, and only depicts direct-current capacitors 105-1, 105-2, . . . , 105-n. In the invention described in Japanese Unexamined Patent Publication No. 2000-152643, so as to determine the life of the direct-current capacitor, a switch SW1, a charging resistance 107, a direct-current-capacitor charge-control circuit 115 are provided to perform initial charge of the direct-current capacitor with a current I obtained by converting the alternating current from the alternating-current power source 103 into a direct current by means of a converter 101. Then, the charge current I detected in the initial charge period of the direct-current capacitors 105 is time integrated by a charge-current integrating circuit 116, and the direct-current-capacitor-capacity estimating circuit 117 calculates an estimated capacity of the direct-current capacitor from the obtained current integration value and the voltage V of the direct-current capacitor. Assuming the capacities of the direct-current capacitors 105-1, 105-2, . . . , 105-n as C1, C2, . . . , Cn, Equation 1 holds true.
                                          C            1                    +                      C            2                    +          …          +                      C            n                          =                              ∫                          I              ⁢                              ⅆ                t                                              V                                    (        1        )            
When the capacity of the direct-current capacitor provided in the direct-current link between the converter and the inverter in the motor driving apparatus decreases by repetition of charge and discharge as described above, the ripple current running through the direct-current link increases, to increase the fluctuation in direct-current voltage, which is problematic. Therefore, it is important to accurately measure the capacity of the direct-current capacitor. Unless the capacity of the direct-current capacitor can be measured accurately, the timing of exchanging the direct-current capacitor may be missed, which has a possibility of causing a large ripple current in the direct-current link or a fluctuation in direct-current voltage, or of exchanging a direct-current capacitor still alive too early. Therefore, it is important to determine whether the life of a direct-current capacitor is ended by accurately grasping the capacity of the direct-current capacitor.
As described above, when there are a plurality of driving shafts to be driven by the motor driving apparatus, a plurality of sets, each of which is made up of a motor, an inverter, and a direct-current capacitor, are provided to respectively correspond to the plurality of driving shafts. In such cases, when attempting to determine the life of a direct-current capacitor based on the invention described in Japanese Unexamined Patent Publication No. 2000-152643, what can be estimated by using the time integration value of a charge current of the direct-current capacitor at the initial charge and the voltage value of the direct-current capacitor and based on Equation 1 is a combined capacity “ C1+C2+ . . . +Cn” of the plurality of direct-current capacitors, but the capacity of each direct-current capacitor cannot be estimated. In other words, a direct-current capacitor having ended its life cannot be identified from a plurality of direct-current capacitors, which is not convenient.